To avoid the use of chlorine as a bleaching agent for pulp or other lignocellulosic materials, the use of ozone in the bleaching of chemical pulp has previously been attempted. Although ozone may initially appear to be an ideal material for bleaching lignocellulosic materials, the exceptional oxidative properties of ozone and its relatively high cost have previously limited the development of satisfactory ozone bleaching processes for lignocellulosic pulps.
Numerous articles and patents have been published related to ozone bleaching of pulp. For example, bleach sequences using ozone are described by S. Rothenberg, D. Robinson and D. Johnsonbaugh, "Bleaching of Oxygen Pulps with Ozone", Tappi, 182-185 (1975)--Z, ZEZ, ZP and ZP.sub.a (P.sub.a -peroxyacetic acid); and N. Soteland, "Bleaching of Chemical Pulps with Oxygen and Ozone", Pulp and Paper Magazine of Canada, T153-58 (1974)--OZEP, OP and ZP. Further, U.S. Pat. No. 4,196,043 to Singh discloses a multi-stage bleaching process utilizing ozone and peroxide with effluent recycle, which also attempts to eliminate the use of chlorine compounds.
Also, various patents disclose vertical bed type reactors for ozone bleaching of pulp in a high consistency range, wherein the pulp is deposited at the top of an essentially quiescent or slowly moving bed and an ozone containing gas is drawn through the bed. For example, Fritzvold U.S. Pat. No. 4,278,496 discloses a vertical ozonizer for treating high consistency (i.e., 35-50%) pulp. Both oxygen/ozone gas and the pulp are conveyed into the top of the reactor to be distributed across the entire cross-section, such that the gas comes in intimate contact with the pulp particles. The pulp and gas mixture is distributed in layers on supporting means in a series of subjacent chambers. The supporting means includes apertures or slits having a shape such that the pulp forms mass bridges thereacross, which the gas passes throughout the entire reactor in contact with the pulp.
Fritzvold et al. U.S. Pat. No. 4,123,317 more specifically discloses the reactor described in the aforementioned Fritzvold '496 patent and Fritzvold et al. U.S. Pat. No. 4,279,694 discloses a method and system for ozone bleaching of pulp using a reactor apparatus as described in the '496 patent. U.S. Pat. Nos. 3,785,577, 3,814,664 and 3,964,962 to Carlsmith each disclose reactor apparatus employing a vertical design similar to the Fritzvold devices, with the '664 patent directed specifically to ozone bleaching. The vertical bed type design described in the preceding patents provides unsatisfactory results with regard to bleaching uniformity.
The ozone bleaching reactor disclosed in European patent application No. 308,314 utilizes a closed flight screw conveyor (an "Archimedes screw") wherein the ozone is pumped through a central shaft and injected into the reactor to treat a layer of pulp that is ideally about 10 cm in height. The pulp has a consistency of 20-50%. European patent application No. 276,608 discloses a further device for ozone bleaching of pulp. In this device a double screw machine, with sections of reverse threads, sequentially compresses and expands the pulp, preferably at 40 to 45% consistency, to provide access of the ozone to the pulp fibers.
Ozone readily reacts with lignin to effectively reduce the amount of lignin in the pulp. But it will also, under many conditions, quickly remove excessive amounts of lignin and aggressively attack the carbohydrate which comprises the cellulosic fibers of the wood to substantially reduce the strength of the resultant pulp. For these reasons, and notwithstanding the various disclosures discussed above, the art generally teaches away from ozone bleaching of pulp at high consistency. For example, Lindholm, "Effect of Heterogeneity in Pulp Bleaching with Ozone", Papieri ja Puu, p.283, 1986, states that the ozone pulp reaction may be "quite heterogeneous" (non-uniform) at pulp consistencies in the range of 30-40%. The heterogeneity is said to be due to part of the pulp receiving greater than average ozone doses while other portions of the pulp do not react at all with the ozone. Also, a recently published Canadian patent application, No. 2,012,771 (published Nov. 10, 1990) discloses a method of bleaching medium consistency pulp with ozone by creating a foam-like mixture of ozone, water and pulp. This application teaches that bleaching at 30% consistency yields worse results than at 10% or 1% consistency due to outer pulp surfaces being overbleached and inside surfaces being unbleached.
A further type of reactor is disclosed in U.S. Pat. No. 4,363,697 to Markham et al. for oxygen delignification of pulp at medium consistency. The Markham device may include a series of screw flights or modified screw flights, with and without paddles, to convey the pulp through a reaction tube in the presence of oxygen. U.S. Pat. No. 4,384,920 to Markham et al. also discloses the use of paddle flights rotated at low speed to convey pulp through the presence of an oxygen gas flow. However, the method disclosed in the Markham patents is generally unsuitable for ozone bleaching reactions due to the much faster reaction rate of ozone and pulp/lignin as compared to that of oxygen and pulp/lignin, and also due to the inability of the device disclosed by Markham to provide uniform gas-fiber contacting and uniform bleaching.
The heterogeneity or non-uniformity problem discussed above may be at least partially overcome by bleaching at medium to low consistency. At medium to low consistency the increased water content allows the ozone to diffuse more evenly through the pulp to increase uniformity. However, the increased water content creates other disadvantages which may outweigh the increased uniformity. The primary disadvantage arises from the increased time required for diffusion of the ozone when there is more water present. This leads to increased ozone decomposition in the water and therefore higher ozone expense as well as poorer bleaching selectivity because of the effects of the ozone decomposition by-products. The result is that at medium to low consistency greater amounts of ozone are required to achieve results equivalent to high consistency bleaching. However, as understood by persons skilled in the art, there is a practical limit on the amount of ozone that can be dissolved in water due to ozone solubility in water. Therefore, it is often not practical or cost effective to attempt to achieve significant increases in brightness with ozone at medium to low consistency.
Another area related to the present invention is the art of conveying, and in particular, with paddle conveyors. The dimensions of flat paddles for use in various diameter paddle conveyors have been standardized by the Conveyor Equipment Manufacturer's Association ("CEMA") in their bulletin ANSI/CEMA 300-1981, entitled "Screw Conveyor Dimensional Standards". Also, Colijn, "Mechanical Conveyors for Bulk Solids" Elsevier, N.Y., 1985, may be referred to as general background in conveying. Although typical prior art conveyors are useful for exposing material to reactive environments or for general blending of bulk solids, and a number of references discussed above use various types of conveyors, prior art conveyors in general are not capable of providing the necessary dispersion of pulp into an ozone containing gas in order to achieve an efficient and uniform ozone bleaching reaction and avoid the problems of the prior art discussed above.